Satellite-retrieved vertical profiles of methane over the Indian region: impact of synoptic-scale meteorology

The altitude distribution of methane (CH4) is the least addressed topic in the greenhouse gas assessment over the Indian region. In the absence of the in-situ measurements, the satellite-based retrievals of the vertical distribution of CH4 using Atmospheric Infrared Sounder (AIRS) measurements during the period 2003-2015 were made use in this study for the first time to understand the 3D distribution (latitude-longitude-altitude) of CH4 over Indian region. Significant regional and seasonal variations are observed in the vertical distribution of CH4, even though it is a long-lived greenhouse gas and known to be well-mixed. Over most of the regions, the highest mixing ratio is observed during post-monsoon months and minimum in the pre-monsoon/monsoon season. The presence of a 'high altitude peak' in CH4 (around 1880 ppbv) around 300 hPa-250 hPa was noted in post-monsoon which is caused by the monsoon-associated convective updrafts and the anti-cyclonic system. The vertical profiles show seasonal variations which are region as well as altitude-dependent. Over the oceanic region, the highest seasonal amplitude of CH4 mixing ratio was observed over North-Arabian Sea due to the proximity of the source rich land regions. During the winter and pre-monsoon months, the latitudinal differences are absent throughout the troposphere. A consistent increasing trend in CH4, ranging from 1 ppbv year(-1) to 6 ppbv year(-1) is seen at all the tropospheric altitudes, with faster growth rates at higher altitudes, maximizing at 300 hPa-150 hPa. An approximate estimate of direct forcing due to CH4 lies in the range 0.80 W m(-2)-0.83 W m(-2). The paper also presents a comparison of the in-situ measured upper tropospheric CH4 mixing ratio from CARIBIC (Civil Aircraft for the Regular Investigation of the atmosphere Based on an Instrument Container) flight data and AIRS retrievals.